Microwave ovens



y 1965 H. BOHM ETAL 3,182,166

MICROWAVE OVENS Filed Aug. 51, 1962 H I H F l G. 2

W 4 @f m -zs x1 52 L INVENTORS HELMUT BOHM HERB RT DURAS BY WM UMAAQA pK, I

ATTORNEYS United States Patent 4 Claims. 01. 219-1055 This inventionrelates in general to microwave ovens and more particularly to apparatusfor insuring equalized heating of foods and other substances exposed tomicrowave energy in the oven.

A major problem of heating an object in a microwave field is the unevendistribution of heat in the object. Unequal distribution of heat canresult from a variety of causes. For example, standing wave patterns setup in the oven enclosure sustain minimum and maximum points of fieldintensity which in turn cause the portions of the object in the moreintense field regions to be heated more than the portions in the minimumfield intensity regions. Also, the object to be heated, such as a mealcontaining several different foods, is electrically nonhomogeneous.Further, objects such as fibers in meats and ice crystals in frozenfoods are electrically anisotropic. Moreover, the electrical propertiesof an object may change during heating, for example, during the thawingof frozen materials, and do so to a different degree in different partsof the object.

One of the prior techniques seeking to obtain a uniform heating ofobjects in ovens employs a rotating turntable which is mounted in aposition asymmetrical with respect to the electrical field. Themicrowave field pat tern in the oven enclosure has minimum and maximumfield intensities in the plane of the turntable as Well as perpendicularto the plane of the turntable. By rotating the turntable, while at thesame time moving the turntable up and down, objects mounted on theturntable will move in a series of helicoidal trajectories and all partsof the objects will move through minimum and maximum points ofelectrical field intensity, thereby evenly heating the objects. Althougheven heating may be obtained with the aforementioned apparatus, it hasbeen found that the two-dimensional drive entails mechanicalcomplications, is continuously in need of maintenance, consumes a largeamount of space, and is apt to be quite noisy.

The primary object of the present invention is to provide a microwaveoven which will evenly heat an object.

Another object of the present invention is to provide a microwave ovenwhich accommodates high power inputs and thereby allows rapid heating ofobjects.

Still a further object of the present invention is to provide amicrowave oven which is compact.

Yet another object of the present invention is to provide a microwaveoven which has a minimum of mechanical movement and need formaintenance.

The invention resides in a microwave oven, having a source of microwaveenergy, the oven having a rotatable dielectric turntable mounted over acylindrical well in the oven enclosure. The bottom surface of the wellis an integral number of quarter wavelengths below the floor of theoven. Covering the well at the level of the oven floor are a pluralityof fixed metallic grids. As the turntable is rotated, objects thereonare uniformly heated by mode patterns of microwave energy set up in theoven.

Other objects and many of the attendant advantages of the invention willbe readily appreciated as the invention becomes better understood byreference to the following 3,132,186 Patented May 4, 1965 detaileddescription when considered in connection with the accompanying drawingswherein:

FIG. 1 depicts a microwave oven in accordance with the invention; and

FIG. 2 depicts a detailed arrangement of the turntable used in the oven.

Referring now to FIGS. 1 and 2, there is shown a microwave oven 2 intowhich objects 4, such as foods, are placed to be heated. A generator 6,conventionally a magnetron tube, feeds microwave energy into a waveguide8 by means of a radiating probe 12. The waveguide in turn transmits themicrowave energy into the oven through a coupling slot 14.

As shown in FIG. 2, a turntable 16, upon which the object to be heatedis placed, is arranged to rotate on a shaft 18. The turntable isconstructed of a dielectric material and has a downturned edge. Directlyin the center of the floor 22 of the oven, as shown in FIG. 2, there isa well 24 which is a quarter wavelength deep. The well is formed by aside wall 26 and a bottom wall 28. A polarizing grid 30 is mounted onthe side wall so that it is flush with the ovens floor and covers thewell. The shaft 18 has a metallic disc 32 which is a half wavelength indiameter attached thereto. The disc is in close proximity to the botomwall 28 of the well, thereby forming a choke joint which preventsmicrowave energy from leaking out the opening in the bottom wall wherethe shaft passes through.

Power to drive the shaft is supplied by a prime mover (not shown)through a shaft 34 keyed to gear 36. Gear 36 meshes with a gear 38 whichdrives the shaft 18. When an object is placed on the turntable 16, theobjects weight causes shaft 18 to move downwardly, causing acounterlever 42 which is attached to the shaft 18 to pivot. Thecounterlever is pivoted on a member 44. Downward movement of the shaftupon one end of the counterlever causes the other end of thecounterlever 42 to move upward and close contacts 48 which thereuponcompletes circuit 52 controlling the power to microwave generator 6.Thus, unless an object is placed on the turntable, the microwave powerto the oven cannot be turned on due to contacts 48 being open. Thedownward movement of shaft 18 need not be appreciable since its onlypurpose is to close contacts 48.

A drain 54, having a hose 56 attached thereto, allows any water formingon the bottom of the well 24 to be drained off. The bottom wall 28 ofthe well is slightly raised at 58 to prevent any water from running downthe shaft opening.

With the foregoing in mind, operation of the device of FIG. 2 is asfollows:

Microwave energy is radiated into the oven through the coupling slot 14and a pattern of standing waves is set up in the oven. A part of thestanding Waves are set up between the bottom and top of the oven. In thevicinity of well 24, microwave energy whose electric field is parallelto the wires of grid 30 will be reflected thereby, while wave energywhose electric field is perpendicular thereto will pass through and bereflected by the bottom wall 28 of the enclosure. Since the well is aquarter wavelength deep, the standing wave pattern of modes reflected bythe polarizing grids 30 and the bottom wall 28 will be displaced withrespect to each other. Further, because grid 30 is flush with the floor22, the standing wave pattern set up by the floor 22 will be continuouswith that set up by the grid.

Consistent with the physical dimensions of the enclosure and the well,the oven will be energized by wave energy in several different modes,thereby setting up a plurality of different standing wave patterns. Eachof the plurality of modes have minimum and maximum points of fieldintensity occurring close together so that it is no longer important asin the past to move the object to be heated up and down, but mererotation of the turntable sufiices to equalize heating throughout theobject to such a degree that thermal conduction is not needed to berelied upon to any considerable extent to ensure even heating, even whenheating is carried out rapidly at high power density.

Therefore, it can be seen that the device allows the use of a small ovenenclosure in which high power density can be obtained at relatively lowpower input levels. While the embodiment shown illustrates the use of avertical movement of the turntable when an object is placed thereupon toactuate a simple on-off type switch, the lever mechanism shown alsoeifectively acts to counter-balance the weight of the object on theturntable.

As the food in the oven is heated, its weight changes because of thevaporization of its water and other volatile substances. Therefore, thesimple on-olf switch 48 can be replaced by a control mechanism whichresponds to changes in weight to control the amount of microwave powersupplied to the oven.

Obviously, many modifications and variations of the present inventionare possible in the light of the foregoing teachings. It is to beunderstood, therefore, that the invention is not limited in itsapplication to the details of construction and arrangement of partsspecifically described or illustrated, and that within the scope of theappended claims, it may be practiced otherwise than as specificallydescribed or illustrated.

We claim:

1. A microwave oven comprising,

an enclosure for confining microwave energy, having top, bottom, andside walls;

a source of microwave energy;

means for coupling energy from the source tothe enclosure;

the bottom wall of the enclosure having a generally cylindrical edgedefining an opening therein;

a well having a wave reflective bottom wall and a side wall, the sidewall of the well connectingthe bottom wall of the enclosure at theopening thereof to the bottom wall of the Well; 7

a polarizing grid disposed over the well, the polarizing grid beingmounted flush with the bottom wall of V the enclosure; and

a dielectric turntable in the oven, the turntable being disposed abovethe well.

2. Apparatus in accordance with claim 1 wherein the distance between thebottom wall of the enclosure and the bottom wall of the well is onequarter Wavelength.

3. Apparatus in accordance with claim 1, and further including a shaftsecured to the turntable, the shaft protruding through the bottom wallof the well and being movable vertically, and means actuated by thevertical movement of the shaft for controlling a circuit permittingpower to be applied to the microwave oven.

4. Apparatus in accordance with claim 3 wherein said shaft protrudesthrough an opening in the bottom wall References Cited by the ExaminerUNITED STATES PATENTS 2,744,990 5/56 Schroeder 219 1o.5s 2,888,543 5/59Haagensen 219 10.55 2,993,973 7/61 Johnson er al 219-10.5s

OTHER REFERENCES German application 1,066,679, printed Oct. 9, 1959.

RICHARD M. WOOD, Primary Examiner.

1. A MICROWAVE OVER COMPRISING, AN ENCLOSURE FOR CONFINING MICROWAVEENERGY, HAVING TOP, BOTTOM, AND SIDE WALLS; A SOURCE OF MICROWAVEENERGY; MEANS FOR COUPLING ENERGY FROM THE SOURCE TO THE ENCLOSURE; THEBOTTOM WALL OF THE ENCLOSURE HAVING A GENERALLY CYLINDRICAL EDGEDEFINING AN OPENING THEREIN; A WELL HAVING A WAVE REFLECTIVE BOTTOM WALLAND A SIDE WALL, THE SIDE WALL OF THE WELL CONNECTING THE BOTTOM WALL OFTHE ENCLOSURE AT THE OPENING THEREOF TO THE BOTTOM WALL OF THE WELL;